Boring machines have been known and used for over 60 years, during which time many improvements have been made on them. These machines are metal cutting machines known to be particularly suited to machine large openings, machine deep surfaces and machine blind surfaces in metal work pieces. They have also been employed to do more conventional machining, i.e. machining readily accessable surfaces. To perform the machining of large openings, and more particularly deep surfaces and blind surfaces, many of the boring machines of the prior art have a rotatable, extendable, cylindrical bar, i.e. boring bar, to support a cutting tool. In some cases the reach of the cutting tool into the workpiece may be many inches, e.g. 20 to 30 inches. The cutting tool is attached to the forward face of the bar, i.e. the face of the bar facing the workpiece, by several methods, e.g. by insertion of a tapered arbor into a mating tapered socket in the face of the boring bar and a draw bar to engage the arbor so as to seat and hold the tapered arbor in place. To rotate the cylindrical boring bar of the prior art machines the boring bar was coupled to gear and/or pulley and belt drive mechanisms by means of a key and keyway arrangement or by a spline. The keyway and spline configurations introduce stress rising points into the boring bar. These stress rising points concentrate stresses introduced into the bar by the torque created during cutting. Concentration of stress at these stress rising points often causes shear failure of the bar at or near the stress rising points. Thus, the cylindrical boring bar having a keyway or spline configuration coupling to a drive means is often weak at the coupling and subject to failure under torque. In machining surfaces deep within a metal workpiece the boring bar is extended from the boring machine toward the workpiece so that the cutting tool reaches the surface to be machined. Thus, extension of the boring bar from the machine in a cantilever fashion aggravates the stress failure problem of the cylindrical boring bar having a keyway or spline configuration for coupling to the drive means for rotating the bar. Not only do the keyway and spline coupling configuration in the cylindrical boring bar of prior art machines act as stress risers but they often also are points of fatigue failure, particularly when the bar is extended from the machine to perform a cutting operation deep within a metal workpiece.
In addition to boring bars having a circular cross-section, i.e. cylindrical boring bars, other shapes have been investigated including a circular cross-section modified by having two diametrically opposed flats and a bar having a body portion having a circular cross-section and at or near one end of the bar a polygon shaped cross-section. Additionally, purely and modified cylindrical bars have no way, by their geometry, of compensating for looseness between them and their mounts which must exist in order for slding to occur and which looseness will increase with time and wear. Accordingly cylindrical bars will rotate in a position excentric to their matching cylindrical supports when supporting radial forces.